There have been numerous studies in which the reproductive cycles of teleosts have been correlated with either environmental cycles or associated physiological cycles, or both. Such correlation is seldom accurately achieved; usually because the reproductive cycle of the species concerned is lax, sometimes because only one or two factors of an integrated whole were examined. The powan of Loch Lomond, Coregonus lavaretus (L. ) (Teleostei, Salmoniformes) is the subject of a long-term study investigating its growth, in particular reproduction. This race is a freshwater glacial relict form of a boreal group, and thus has an exceptionally strictly times reproductive cycle. It thus represents an ideal subject for cyclical studies. This thesis investigates the relationship between lipid storage and the reproductive cycle and the role of thyroid gland.